The present invention relates to a converter that converts AC (alternating current) power into DC (direct current) power and a power converter that becomes it with the converter to produce AC power having any frequency and voltage.
In a driving system of an electric motor, it is necessary to change a frequency of AC power to any frequency in order to control a rotational speed of the motor freely and a power converter using semiconductor switching devices such as insulated gate bipolar transistors (IGBTs) is used therefor. Most of such power converters each include a converter for converting AC power into DC power and an inverter for converting the DC power produced by the converter into AC power having any frequency and voltage.
However, since a DC voltage is varied due to unequalness of the DC power transferred between the converter and the inverter, a smoothing condenser is provided in a DC circuit connecting therebetween to suppress the variation of the DC voltage. Generally, an electrolytic condenser used as the smoothing condenser is very large in the volume as compared with semiconductor switching devices and accordingly the percentage of the volume of the smoothing condenser occupying the power converter is increased. Therefore, if the capacity of the smoothing condenser is reduced, the power converter can be made small.
Heretofore, in order to reduce the capacity of the smoothing condenser, Japanese Patent Publication JP-A-2003-102177, for example, discloses that an input current and a DC input current based on a current command for an inverter are presumed and an AC input current of the converter is controlled so that a DC output current of a converter is equal to the DC input current of the inverter. Further, Japanese Patent Publication JP-A-7-79567 discloses that an active filter is connected in parallel to a smoothing condenser and a compensation current of the active filter is controlled so that DC voltage variation is absorbed.